Reverse Spherification – Making Food Pearls

Spherification in molecular gastronomy is the process of taking a liquid and turning it into a ball of jelly with the centre still in liquid form. Spherification is achieved by one of two methods, “Spherification” and “Reverse Spherification”. For this post I’ll be focusing on Reverse Spherification because it is the one I used to make the grapefruit and raspberry pearls for my tasting menu. See the recipes here: Langoustine and Lobster Cannelloni with Grapefruit Pearls and Chocolate Cremeux with Raspberry Pearls.

Basic spherification involves dropping a liquid mixed with sodium alginate into a calcium bath (generally calcium chloride). This creates a gel from within the sphere so there is no real skin, it bursts easily in the mouth without a jelly like mouth feel. But basic spherification has two major limitations. First, the jellification process doesn’t stop once the sphere is removed from the calcium bath. This means the spheres must be made and served immediately or you will just end up with a ball of flavoured jelly. Second, it is not suitable for all ingredients. Highly acidic liquids or anything with a high alcohol content or containing calcium will not work. You can add sodium citrate to correct the PH level but sodium citrate will alter the taste of your sphere.

Reverse spherification involves adding a calcium such as calcium gluconate (which is tasteless) to a liquid and then dropping the liquid into a sodium alginate bath. The advantage of this method is the ability to use almost any liquid, high calcium liquids probably won’t need calcium gluconate and the acidity / alcohol content of the liquid won’t matter either. So if you want cocktail spheres, reverse spherification is the way to go. When you remove the spheres from the sodium alginate bath and rinse them in water they will stop jellifying. Spheres made using reverse spherification will keep for several days, can be made to the desired thickness and since the skin of the sphere is permeable it can be left in a flavoured liquid to marinate. The downside is that the skin of the sphere will be thicker, so you while you will get the satisfying burst of liquid, you will feel the thin jelly skin.

Using reverse spherification is slightly trickier than your basic spherification method. The majority of the challenges come from using a sodium alginate bath… it thickens the water; too thick and your drops of liquid will end up as pancakes on the surface of the bath, too thin and the droplets will disperse before it forms a proper sphere. The spheres will also stick to each other in the bath, so they must be kept apart – limiting the number you can do at one time. You will also need to ensure proper dispersion of the sodium alginate into the water, that is usually done with an immersion blender. It takes awhile to get the bath properly mixed and when you are done you will notice air bubbles trapped in the bath. You will need to let the bath rest for at least 12 hours before you can use it, otherwise the air bubbles will damage or weaken your spheres.

Preparing your liquid:

Take your liquid that you are about to spherify and measure out 2% calcium gluconate or 1% calcium lactate by weight. Do not, use calcium chloride as you would in basic spherification, it is salty and bitter and will completely change the taste of your spheres. Take your calcium and add it to about half of your liquid, mix it in with an immersion blender, then add the remaining half of the liquid and blend until completely dissolved. If your liquid is already high in calcium (like milk) then it won’t need additional calcium gluconate added.

Now check the thickness of your liquid. It needs to be about the thickness of double cream, if it is too watery and thin it will just splash on the surface of the alginate bath. If you need to thicken your liquid you should use xanthan gum. How much xantham gum you add depends on how much thicker the liquid needs to be… 0.5% is probably a good start. Hit it with the immersion blender and leave it to rest for about 20mins then check again. Repeat until you reach the desired thickness. If your liquid foams up, as it did with my grapefruit pearls, strain in through a sieve. That will help remove the air bubbles and should get the foam back to a liquid like state.

Preparing the bath:

You will need 0.5% sodium alginate to distilled water by weight. It is important that you do not use tap water, it will contain calcium which will react with the sodium alginate and the whole thing will become a gel. I don’t recommend mineral water for the same reasons. Cold, distilled water is required, either make it yourself or buy it from a hardware / car parts store.

Sodium alginate is actually quite hard to disperse and hydrate. You can mix a small amount of sugar with the alginate powder to help dispersion. Mix the alginate and sugar into half of the distilled water with an immersion blender, until it has completely dissolved. Even with an immersion blender, it will take a good few minutes to completely dissolve the alginate. Add the rest of the water and mix for another few minutes before allowing the bath to rest for about 12 hours. You can speed up the process by straining it through a fine sieve or chinois.

Forming the spheres:

Now comes the tricky part, making your spheres. You should setup a system of bowls or trays. One with the liquid, another with the alginate bath, another bowl with just tap water and finally a plate to serve or a bowl filled with juice or other flavouring to store the spheres. It is important that you do not transfer any alginate into the calcium prepared liquid. If you do then it is likely that the entire liquid will end up as a jelly; so keep your utensils completely separate! There are two ways to get the right shape. The easy way is to freeze your liquid in small balls using a special freezer tray, that will produce perfect results every time. The other, more common way is to lower the liquid in with a spoon. It is best to use a rounded spoon, like a small measuring spoon. Take a spoonful of liquid and keep it just above the alginate bath – too high and the liquid will spread but you don’t want to put the spoon in the bath. Carefully but quickly, pour the liquid into the bath, do a couple at a time. I recommend a flat-bottomed tray or dish for the alginate bath, if the spheres sink they need to be kept apart, a curved bottom will have them all bunch together. You don’t want the spheres sitting at the bottom anyway, one side will end up flat and with a thinner skin that the rest, so give them a stir. If the spheres float, you need to stir the bath to ensure that the spheres are “cooked” on all sides equally, splashing the spheres with alginate water also helps.

If a few spheres burst or tiny drops split off and form little spheres then you will need to clean out the bath. You can remove larger bits with a slotted spoon but if there is a build up of gunk just strain the bath with a sieve and carry on.

Depending on how thick and stable you want your spheres you will need to let them cook in the bath for about two and half minutes. Then remove them from the bath with a slotted spoon and let them rest in a bowl of clean water for about 30 seconds (try not to let them stick together) before serving or reserving in a tub of juice or other flavouring. I do not recommend leaving them in a tub of water, as the sphere is permeable it will become tasteless. The easiest thing to do is keep the pearls in a tub of their own juice – they will store like this for a few days so they can be made in advance.

Another, easier option for forming the spheres is to freeze the liquid first. You need something like a mould to freeze them in small ball shapes but that way you don’t need to worry about thickening the ingredient. Just drop the frozen balls in and allow the skin to form. If you have any questions on spherification just let me know in the comments below.

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You’re welcome Walter. By “Calcium Gluconate” I mean Calcium Gluconate Lactate, usually referred to as Gluco from the El Bulli range, so it’s 2% by weight.

Alcohol shouldn’t change the required amount of calcium to spherify, unless you use an alcohol with cream in it – like Baileys or Coole Swan. I’ve never tried tequila spheres so I’m not sure how much xanthan gum would be required, I would start around 0.8% as that is usually a good default starting point. One thing you could try would be to freeze the tequila and gluco mix. Even though alcohol doesn’t freeze, the water in the alcohol will cause the mix to thicken a little (bit like when you freeze vodka). That might reduce the need for as much xanthan.

My final recommendation when making alcohol spheres is to mix it with water. Pure tequila in a sphere will be very strong, it will cause people to cough when they burst. It’s not so bad when making a mixed cocktail sphere (like a B52) but using purely strong alcohol (40%+ abv) will be too much! Spherification gives you an intense burst of flavour so you need to tone it down a little when using alcohol. I would suggest starting with 33% water to tequila and working from there. If you want to enhance the tequila flavour without making the sphere too alcoholic you can try adding agave syrup. Good luck!

Thanks for this post on spherification. You said that you use a 2% calcium gluconate or 1% calcium lactate by weight in the liquid you want to spherify. What about if you are using calcium lactate glutonate (aka calcium gluconolactate)? What would the percentage by weight be then? Also, I’ve read that liquids with alcohol in them work well for reverse spherification…does the percentage of calcium that is added to the alcoholic liquid change or does it stay the same? For example, if I wanted to make a tequila sphere, what would the measurements be for tequila, calcium lactate glutonate, and xantham?

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